The invention relates to a method by which an adherent platinum coating can be applied to hard, high density refractory substrates.
Various methods of metallically coating refractories have been previously disclosed. See U.S. Pat. No. 3,598,635. In order to achieve bonding of the coating to the substrate it has been necessary to initially roughen the surface of the refractory by grit blasting or chemical etching.
U.S. Pat. No. 3,310,433 discloses a method for coating ceramics of alumina or zirconia by either slip coating with a ceramic layer or grit blasting the ceramic to create a roughened area sufficient to apply by dipping, spraying or painting a molten silver coating saturated in oxygen to the ceramic body.
U.S. Pat. No. 3,436,203 describes a coated refractory for contacting molten glass by lining the refractory with an electroconductive metal oxide having an electrical resistivity that is selectively matched to a noble metal coating which is applied thereon. The disclosure teaches that such an arrangement provides an interfacial electrical contact potential between the metal oxide coating and the noble metal coating which maintains a low contact electrical potential when in contact with molten glass. This low contact potential is designed to overcome the high contact potential created when molten glass seeps through minute openings in the noble metal clad refractory, thereby causing seeds or blisters to be introduced into the molten glass. The preferred method disclosed therein for coating the refractory was flame spraying both the metal oxide and the noble metal onto the refractory.
U.S. Pat. No. 2,777,254 discloses the method of coating a refractory of zirconium silicate with platinum foil which requires coating the refractory initially with a thin layer of alumina which is dusted upon the refractory surface and then applying thereto a thin platinum foil over the refractory body which is then fired to a temperature of 3030.degree. F. A weighted refractory is used to exert pressure upon the foil during firing and is removed upon cooling. An aluminum zirconium silicate glass bond is said to result which causes adherence of the platinum to the refractory material.
I have found, however, that the platinum coating applied to grit blasted or chemically etched surfaces of hard, high density refractories have in many instances separated from the refractory upon cooling due to the inadequate roughening of the refractory surface to which the platinum coating must adhere. Neither grit blasting or chemical etching such hard dense refractories has sufficiently roughened the surface to allow an adherent mechanical bonding of the platinum to the refractory.